The present invention relates to plants for the production of asphalt, and, in particular, to a plant that can be operated both in a batch mode and in a continuous mode.
In general, asphalt plants are made to operate only in a batch mode or only in a continuous mode. In a batch mode, the constituents of the asphalt product are carried by a bucket elevator into a batch tower, are individually weighed, are loaded into a mixer as a batch, are mixed together, and then are put into a silo for storage. In a continuous mode, the constituents of the asphalt product are continuously introduced into the mixer in the proper proportions and are moved through the mixer as it operates, so that the product continuously leaves the mixer. In general, a plant designed for continuous mode operation does not include a bucket elevator.
Some plants have been made to function both in a batch mode and in a continuous mode. This flexibility has been achieved by adding a rotary mixer to the traditional batch plant and feeding the mixer in a continuous mode or feeding the batch tower in a batch mode, as desired. This combined system is popular, due to the flexibility it adds to a traditional batch plant. However, there has been a shortcoming with known combined continuous/batch systems in that they cannot be "hot-stopped"--that is, they cannot be stopped for a temporary pause during operation.
There is a need to be able to "hot-stop" the combination plant when it is operating in continuous mode, for example, in the event that there is an emergency or in the event that a silo becomes full, while the operator knows that trucks are coming soon to unload the silo, and it would be desirable not to shut down the plant.
However, it has not been possible to "hot-stop" prior art combined continuous/batch plants, because they did not have controls that were sophisticated enough to control the mix of materials when production was suspended, and because they were not capable of stopping and starting under load. For example, in prior art combined plants, if the bucket elevator were stopped while the buckets were full of material, the elevator would reverse, dumping material at the bottom of the elevator. So, in prior art combined plants, a "hot stop" meant, among other things, that two people with shovels would have to spend an hour cleaning out the mess that was made when the buckets dumped their loads. Of course, once the buckets dumped their loads, it would take some time before the product coming out of the plant would be according to specifications, so the start-up after the prior art "hot stop" included the production of a substantial amount of waste. Also, in prior art combined plants, the rotary mixers are generally driven by trunnion drives, which, if stopped under load, have a great tendency to slip in trying to start back up again.
Another shortcoming of prior art combined system plants is that they waste a large amount of material during start-up and shut-down when operating in continuous mode. This happens, because it takes a period of time for the constituents of the asphalt to reach the right proportions as the plant is starting up, and everything that is produced before that time is wasted. Also, as the plant shuts down, the constituents of the asphalt stop entering the mixer in the right proportions, so everything produced thereafter is wasted.
Another shortcoming of prior art combined system plants is that they tend to create bad product when they are being shifted from one production level to another, for the same reasons cited above, again creating waste.
Another shortcoming of prior art plants is that they waste energy, because they use a damper to control the flow of fugitive emissions from the mixer to the burner, so the fugitive emissions fan constantly draws a high horsepower, even when the damper has been closed down to reduce the flow of air.
Another shortcoming of prior art plants is that there is turbulence as the fugitive emissions are introduced to the burner, which interferes with the burner flame.